Process of producing hydrofluoric acid low in fluosilicic acid



Sept. ,15, 1942. l.. PRElsMAN PROCESS 0F PRODUCING HYDROFLUORIG ACID LOW IN FLUOSILICIC ACID ATTORNEY Patented Sept.A 15, 1 942 PROCESS OF PRODUCING .HYDROFLUORIC- ACID LOW IN FLUOISILICIC ACID Louis Breisman, Cleveland, Ohio, assignol` to General Chemical Company, New York, N. Y., a corporation of New York Application April 8, 1940, Serial No. 328,477

7 Claims.

This invention relates to a process of preparing hydrofluoric acid substantially free, or having a low content of iiuosilicic acid.

One commonly` used commercial method of making hydrofluoric acid involves the treatment of "uorites such as iluorspar with concentrated silica content, below 1.0% have been used. Such fluorites are diicult to obtain, and furthermore, are expensive. Fluorites. containing up to about 3.0% silica, on the other hand, are plentiful and comparatively inexpensive.

United States Patent 1,875,200 of August 30, 1932, discloses the treatment of iluorites having a relatively high content of silica by treatment of the iluorite with sulfuricacid of a concentration of between 70 and 90% H2804 and heating the resultant mixture to a temperature of from 70 to 120 C. The specication of this patent points out that to obtain a hydrofluoric acid ywhichyis as anhydrous as possible after treatment of the fluorite with sulfuric acid of a concentration between 70 and 90% H2804, the concentration of the acid is increased by the addition of fuming sulfuric acid. In carrying out the process of this patent it was found that the best removal of silica obtainable was of the order of.

33%.of `the totalsilica content originally present in the fluorite, i.` e; `a reduction of 33% in the silica content of the raw material was possible. Further, the process of the patent `re` quired very 'careful temperature control,` a facand which is diicult to hande, renders the process of this 'patent n otleadily adaptable to the commercial production of hydroiluoric acid.

It is an object of thisfinvention to provide a process of producing. hydroiluoric acid resulting,

" in an acid substantially free of or low in uosilicio acid from iluorites relatively high in Ipug mixer, to produce a damp and crumbly not sticky mix, and then heatingthis mix in a readily vbe carried out in a comparatively inexpensive manner and results in a reduction of the silica content of the uorite `to a substantially greater extent than is possible by the process of United States Patent 1,875,200. Another object of the invention is to provide a process of producing hydrofluoric acid substantially free or having a low content of fiuosilicic acid, which process'results in the production of fluosilicic acid or sodium. fluosilicate as a by-product. Other objects and'advantages of this invention will appear from the following detailed description thereof.

I have made the surprising discovery that the silica content of `lluorites such /as fluorspar containing up to 3.0% silica can be reduced to a substa'ntialy greater extent than is possible by the process of said patent by treating the lluorite with hydrofluoric acid ofa concentration of between .30 and 40%, preferably about 36%, in amount so that the weight ratio of hydrofluoric acid to silica (SiOz) is equivalent to from 1:1 to 2.5:1. In order to economize `on the amount of hydroluoric acid employed in -the process preferably the weight ratio or acid to silica used is about 1.67 :1; withthis-ratio of acid to silica, Lhave found, the silica content can be reduced as much as about and with a higher ratio, e. g. 2.5:1, the silica content can be reduced as much as The treatment may be carried out by mixing the fluorite containing silica in excess of 1.0% with the hydroiiuoric acid in the concentration `and amount stated, for example, a ut furnace to a. temperature of approximately-200 C. for one to two hours. 4The gases produced in this heating step, constituted of SiF4, HF, H2O

are scrubbed with a circulating solution of fluo- *silicio acid of the same composition as that produced by condensing the gases. Afluosilicicfacid of about 35% concentration is thus obtained.

This acid may be reacted with soda ashy toform sodium uosilicate.

',As above indicated, the treatment hereinabove l described, involving the use of a weight ratio of hydrofluoricacid to silica of 1.67z1, results in a reduction of the'silica content of .the uorites of the order ofrvabout 70%, producing, in the case of fluorites'c'ontaining up to about 3.0% silica, a. fluori'tev containing less thanV about 1.0% S102.

. This fluorite may be treated with-sulfuric acid. in

silica, e. g. up to about.'3.0%, which process can 55 accordance with the usual procedure to produce hydroiiuoric acid' substantially free of or low in iiuosilicic acid.

The accompanying drawing illustrates a preferred layout of apparatus for practicing the process of this invention.

In this drawing, reference numeral I identifies a hopper for the receptiorLof ground fluorspar or l other uorite; this hopper is provided with a valve controlled outlet 2 communicating with a weighing hopper 3. Hopper 3 supplies` a weighed charge of iiuorite to a mixer 4. A storabsorption tower I5. Fluosilicic acid is circulated through the absorption tower I5 by means of a pumpv I6 which pumps fluosilicic acid from tank I1 provided with a cooling coil I8 'through pipeline I9 whichsupplies the acid to a distributor head 2| in the absorber I5. Fluosilicic acid solution flows from the bottom of the absorber I5 through line 20 into the tank I'I. rection of iiow through the absorber I5, tank I1 and connecting lines is indicated on the'drawing by the arrows associated'with these parts.

Uncondensed gas leaves the absorber I5 through pipeline 22 which may connect with a fan for drawing the gas through the system. vF'luosilicic acid may be withdrawn as desired from the circulating stream through the valve controlled takeoff pipe 23 and passed to storage or treated with soda ash to form sodium fluosilicate. i k

The following example illustrates a preferred embodiment of the invention; it will be understood that this inventionis not limited to this example.

Ground iiuorspar containing about 2.3% silica was mixed with 36% hydrouoric acid in amount equal to about 217 pounds of acid per ton of spar, the weight ratio of acid to SiO: being equivalent to 1.67z1. A pug mixer was used for this purpose and produced a damp, crumbly, butr not sticky, mix. This'mix was then fed 4to an externally red rotary furnace where it was heatedto approximately 200 C. for 1 to 2 hours. Approximately 70% of the silica content was driven oi during this heating, the silicon tetrauoride passing through the gas exit line along with unreactedacid, water vapor, and usually some carbon dioxide. Calcium fluoride, containing not excess of 1.0% but not more than about 3.0% silica, which comprises treating the uorite with hydrouoric acid of between 30 to 40% concentration, in amount such that the weight ratio of hydrofluoric acid to silica present in the fluorite is within the range of from 1:1 to 2.5:1, by heating the mixture of fluorite and hydrofluoric acid to drive off silicon fluoride, thereby reducing the silica content so that the fluorite contains less than about 1.0% silica, and treating the resultant fluorite .with sulfuric acid to produce hydrofluoric acid low in fluosilicic acid.

2. A process of producing hydroiiuoric acid from fluorspar containing in excess of 1.0% but not more than about 3.0% silica, which comprises adding hydrofluoric acid of 30 to 40% concentration to the fluorspar to produce a mixture having a weight ratio of hydroiiuoric acid to silica.

of about 1.67 :1, heating the resultant mixture to a temperature of about 200 C. to drive o silicon fluoride and produce a calcium fluoride The diresidue containing less than 1.0% silica, and treating this residue with sulfuric acid to produce hydroiluoric acid.

3. A process of producing hydrofluoric acid from fluorspar containing in excess of 1,0% but not more than about 3.0% silica, which comprises adding hydrofluoric acid of about 36% concentration to the fluorspar to produce a mixture having a weight ratio of hydrofluoric acid to silica4 of about 1.67z1, heating the resultant mixture to a temperature of about. 200 C. Vto drive oif silicon fluoride and produce a calcium fluoride residue containing less than 1.0% silica, and

treating this residue with sulfuric acid to pro-l duce hydroluoric acid.

4. A process of producing hydroiiuoric acid from fluorites containing in excess of 1.0% but not more than about 3.0% silica, which comprises treating the fluorites with hydroiiuoric acid of 30 to 40% concentration in amount such that the weight ratio of hydrouoric acid to silica present in the iluorites is within the range of from 1:1 to 2.5:1, heating the resultant mixture to drive off silicon iiuoride, condensing the exit gases by absorbing them in a fiuosilicic acid solution to produce fiuosilicic acid, and treating the reaction mixture from which the silica has been driven oif with sulfuric acid to produce hydroiluoric more than about 0.8% silica, was discharged from the furnace and later treated with concentrated sulfuric acid to produce hydroiiuoric acid containing about 1.8% fiuosilicic acid.

The exit gases from the furnace, after being cleaned by passage through a dust collector, were scrubbed with a 35% uosilicic acid solution.

Fluosilicicacid of 35% concentration resulted.

This acid can be disposed of as such as lasalable product or reacted with soda ash to form so" diurn fiuosilicate. l

Itis to'be understood that this invention is not restricted to the present disclosure otherfwise'than as defined by the appended claims.

What is c laimed is:

acid.

5. A process of producing hydroiiuoric acid,

and, as a by-product, fluosilicic acid, from fluorspar containing in excess of 1.0% but not more than about 3.0% silica, which comprises mixing hydrofiuoric acid of about 36% concentration with the fluorspar in such" proportion that the weight ratio of hydrofluoric acid to silica is about 1.6721, heating the resultant mixture for one to two hours at a temperature of 200 C. to drive off silicon fluoride and produce a calcium fluoride containing less than 1.0% silica, condensing the exit gases from the aforesaid heating step by absorbing them in a 35% iluosi1icic acid solution to produce fluosilicic acid of about 35% concentration, and treating the calcium uoride containing less than 1.0% silica with sulfuric acid to produce hydrofiuoric acid.

6. A process of reducing thef'silica content of fluorites containing in excess of 1.0% but not more than about 3.0% silica to a value below 1.0%, which comprises mixing the fluorites with hydroiluoric4 acid of a concentration of between .30 and 401%, in amount such that the weight ratio of hydrofluoric acid to silica is withinthe range of from 1:1 to 2.5:1 and heating the resultant mixture to drive oil! silicon fluoride and weight ratio of hydrouoric acid to silica is equal to about 1.67:1, and heating this Amixture to a temperature of about 200- C. for one to two hours to reduce thevsilica content to a value be- 5 low 1.0%.

LoUIs PREISMAN. 

